Surface stabilized electrodes for lithium batteries

ABSTRACT

A method of stabilizing a metal oxide or lithium-metal-oxide electrode comprises contacting a surface of the electrode, prior to cell assembly, with an aqueous or a non-aqueous acid solution having a pH greater than 4 but less than 7 and containing a stabilizing salt, for a time and at a temperature sufficient to etch the surface of the electrode and introduce stabilizing anions and cations from the salt into said surface. The structure of the bulk of the electrode remains unchanged during the acid treatment. The stabilizing salt comprises fluoride and at least one cationic material selected from the group consisting of ammonium, phosphorus, titanium, silicon, zirconium, aluminum, and boron.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 11/809,133, filedMay 30, 2007, which claims the benefit of provisional application Ser.No. 60/809,478 filed May 31, 2006, each of which is incorporated hereinby reference in its entirety.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. DE-AC02-06CH11357 between the United States Government andUChicago Argonne, LLC representing Argonne National Laboratory.

FIELD OF THE INVENTION

This invention relates to improved metal-oxide- and lithium-metal-oxideelectrodes for lithium cells and batteries, notably rechargeablelithium-ion cells and batteries. These batteries are used to power awide range of applications such as portable telecommunicationsequipment, computers, medical devices, electric vehicles andhybrid-electric vehicles. The invention relates preferably tolithium-metal-oxide electrodes with layered- and spinel-type structuresthat are chemically preconditioned prior to cell assembly or in-situ inan electrochemical cell to improve the capacity, cycling efficiency andstability of lithium cells and batteries when charged to highpotentials.

BACKGROUND OF THE INVENTION

State-of-the-art lithium-ion cells have a lithiated carbon negativeelectrode, or anode, (Li_(x)C₆) and a lithium-cobalt-oxide positiveelectrode, or cathode, Li_(1−x),CoO₂. During charge and discharge of thecells, lithium ions are transported between the two host structures ofthe anode and cathode with the simultaneous oxidation or reduction ofthe host electrodes, respectively. When graphite is used as the anode,the voltage of the cell is approximately 4 V. The LiCoO₂ cathode, whichhas a layered structure, is expensive and becomes unstable at lowlithium content, i.e., when cells reach an overcharged state at x>0.5.Alternative, less expensive electrode materials that are isostructuralwith LiCoO₂, such as LiNi_(0.8)Co_(0.2)O₂, LiNi_(0.5)Mn_(0.5)O₂ andLiMn_(0.33)Ni_(0.33)Co_(0.33)O₂ are being developed with the hope ofreplacing at least part of the cobalt component of the electrode.However, these layered structures, when extensively delithiated becomeunstable, because of the high oxygen activity at the surface of theparticles. Therefore, the delithiated electrode particles tend to reactwith the organic solvents of the electrolyte or lose oxygen. Suchreactions at the surface of metal-oxide-and lithium-metal-oxideelectrodes, in general, are detrimental to the performance of thelithium cells and batteries, and methods are required to combat thesereactions to ensure that maximum capacity and cycle life can be obtainedfrom the cells.

Several efforts have already been made in the past to overcome thestability and solubility problems associated with lithium-metal-oxideelectrodes. For example, considerable success has been achieved bystabilizing electrodes by pre-treating the electrode powders with oxideadditives such as Al₂O₃ or ZrO₂ obtained from metal alkoxide precursorssuch as solutions containing aluminum ethylhexanoate diisopropoxide(Al(OOC₈H₁₅)(OC₃H₇)₂ or zirconium ethylhexanoisopropoxide(Zr[(OOC₈H₁₅)₂(OCH₃H₇)₂]) as described, for example, by J. Cho et al inChemistry of Materials, Volume 12, page 3788 (2000) and J. Cho et al inElectrochemical and Solid State Letters, Volume 4 No. 10, page A159(2001), respectively, or a zirconium oxide, polymeric precursor orzirconium oxynitrate (ZrO(NO₃)₂.xH₂O) as described by Z. Chen et al inElectrochemical and Solid State Letters, Volume 5, No. 10, page A213(2002), prior to the fabrication of the final electrode thereby makingthe surface of the LiCoO₂ particles more resistant to electrolyteattack, cobalt dissolution or oxygen loss effects.

The loss of oxygen from lithium metal oxide electrodes, such as layeredLiCoO₂ and LiNi_(1−y)Co_(y)O₂ electrodes can contribute to exothermicreactions with the electrolyte and with the lithiated carbon negativeelectrode, and subsequently to thermal runaway if the temperature of thecell reaches a critical value. Although some success has been achievedin the past to improve the performance of lithium-ion cells by coatingelectrode particles, the coatings can themselves impede lithiumdiffusion in and out of the layered electrode structure duringelectrochemical discharge and charge. Further improvements in thecomposition of high potential metal-oxide- and lithium-metal oxideelectrodes, particularly at the surface of the electrodes, and inmethods to manufacture them are still required to improve the overallperformance and safety of lithium cells.

Lithium metal oxides that have a spinel-type structure are alternativeelectrodes for commercial lithium-ion cells and batteries, notably thoseused in high-power applications. Of particular significance is thelithium-manganese-oxide spinel, LiMn₂O₄, and its cation-substitutedderivatives, LiMn_(2−x)M_(x)O₄, in which M is one or more metal ionstypically a monovalent or a multivalent cation such as Li⁺, Mg²⁺ andAl³⁺, as reported by Gummow et al. in U.S. Pat. No. 5,316,877 and inSolid State Ionics, Volume 69, page 59 (1994). It is well known thatLiMn₂O₄ and LiMn_(2−x)M_(x)O₄ spinel electrodes are chemically unstablein a lithium-ion cell environment, particularly at high potentialsand/or when the cell operating temperature is raised above roomtemperature, when manganese ions from the spinel electrodes tend todissolve in the electrolyte. This process is believed to contribute tothe capacity loss of the cells at elevated temperatures. Moreover, theremoval of all the lithium from LiMn₂O₄ and LiMn_(2−x)M_(x)O₄ electrodesyields a MnO₂ component, which itself is a strong oxidizing agent. Thesurface of such delithiated spinel electrodes can have a high oxygenactivity, thereby possibly inducing unwanted oxidation reactions withthe electrolyte. Although considerable progress has been made tosuppress the solubility and high-temperature performance of spinelelectrodes and to improve their stability by cation doping, as describedfor example by Gummow et al. in U.S. Pat. No. 5,316,877, or by formingoxyfluoride compounds as described by Amatucci et al. in the Journal ofthe Electrochemical Society, Volume 149, page K31 (2002) and by Choi etal. in Electrochemical and Solid-State Letters, Volume 9, page A245-A248(2006), or by surface coatings as described by Kim et al. in the Journalof the Electrochemical Society, Volume 151, page A1755 (2004), thesetreatments have not yet entirely overcome the cycling instability ofcells containing manganese-based spinel electrodes.

Furthermore, other metal-oxide- and lithium-metal-oxide electrodematerials that are good oxidants are of interest for lithium batteriesare known, for example, V₂O₅, and materials containing a V₂O₅ component,such as LiV₃O₈ and AgV₃O₈, that can be written alternatively intwo-component notation as Li₂O.3V₂O₅ and Ag₂O.V₂O₅, respectively, andAg₂V₄O₁₁ that can be written alternatively in two-component notation asAg₂O.2V₂O₅. The silver-containing materials, notably Ag₂V₄O₁₁, are ofparticular interest for primary lithium cells in medical devices such ascardiac defibrillators. In this case, a preconditioned electrode with astable surface layer will help prolong the life of the cell,particularly if left standing in the charged state or partially chargedstate for long periods of time. The invention extends to include MnO₂and MnO₂-containing compounds which, like V₂O₅, are strong oxidants,such as Li₂O.xMnO₂ and Ag₂O.xMnO₂ (x>0) electrode compounds.

It is clear from the prior art that further advances are required, ingeneral, to improve the surface stability of metal-oxide andlithium-metal-oxide electrodes for non-aqueous lithium cells andbatteries. This invention relates to such improvements, notably thosethat are achieved from stabilized electrode surfaces that are engineeredby preconditioning electrode particles with aqueous or, preferably,non-aqueous solutions in which the dissolved salts contain bothstabilizing cations and anions. The invention relates more specificallyto uncycled, preconditioned metal oxide- or lithium metal oxideelectrodes, the electrodes being preconditioned in an aqueous or anon-aqueous solution containing stabilizing cations and anions, suchthat the stabilizing ions are etched into the electrode surface to forma protective layer. Methods of preconditioning the electrodes aredisclosed as are electrochemical cells and batteries containing theelectrodes.

SUMMARY OF THE INVENTION

This invention relates, in general, to improved metal-oxide andlithium-metal-oxide electrodes, including cathodes and/or anodes forlithium cells and batteries, preferably rechargeable lithium-ion cellsand batteries. More specifically, the invention relates to metal-oxideand lithium-metal-oxide electrodes that are chemically preconditionedprior to cell fabrication and assembly or in-situ in an electrochemicalcell by treating the electrode particles with an aqueous or anon-aqueous solution containing dissolved salts of both stabilizingcations and anions. The invention relates more specifically to electrodeparticles with surfaces that are simultaneously etched and protected bythe solutions, preferably, but not necessarily, mildly acidic solutionscontaining stabilizing ammonium, phosphorus, titanium, silicon,zirconium, aluminum and boron cations and fluoride anions, such as thosefound in NH₄PF₆, (NH₄)₂TiF₆, (NH₄)₂SiF₆, (NH₄)₂ZrF₆, (NH₄)₃AlF₆, NH₄BF₄or derivatives thereof, optionally in the presence of lithium ions toform a protective surface layer on the electrode particles, therebyimproving the capacity, cycling efficiency and cycling stability oflithium cells and batteries when charged to high potentials. Theinvention relates, in particular, to high potential metal oxide- andlithium-metal oxide electrodes that in the charged state are strongoxidants, for example, those selected from the family of charged layeredelectrodes, Li_(1−x)MO₂, and those derived from lithium-richLi_(1+z)M_(1−z)O₂ compounds. Such lithium Li_(1+z)M_(1−z)O₂ compoundscan also be represented in two-component notation as xLi₂M′O₃.(1−x)LiMO₂(0≦x<l) in which M′ is one or more metal ions with an averagetetravalent oxidation state and in which M is one or more metal ionswith an average trivalent oxidation state. It stands to reason that theinvention will also apply to other high-potential metal oxide andlithium-metal oxide electrodes such as spinel lithium-metal-oxides,LiM₂O₄, in which M is one or more metal cations with an averageoxidation state of 3.5. The spinel electrodes are selected preferablyfrom the subset of substituted spinel lithium-manganese-oxidesLiMn_(2−y)M_(y)O₄, and two-component ‘layered-spinel’xLi₂M′O₃.(1−x)LiM₂O₄ (0≦x<1) composite electrodes in which M′ is one ormore metal ions with an average tetravalent oxidation state, asdescribed above. The invention also applies to the family ofV₂O₅-containing compounds, such as V₂O₅ itself, and lithium- andsilver-derivative compounds such as LiV₃O₈ (Li₂O.3V₂O₅) and Ag₂V₄O₁₁(Ag₂O.2V₂O₅) and to MnO₂, and MnO₂-containing compounds, such asLi₂O.xMnO₂ and Ag₂O.xMnO₂ (x>0) electrode compounds. The inventionextends to methods for synthesizing the preconditioned metal-oxide andlithium-metal-oxide electrodes.

The principles of the invention are demonstrated with respect to thefollowing samples:

-   -   Sample A: untreated        0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂    -   Sample B: Sample A treated with 0.86 wt % NH₄F in methanol    -   Sample C: Sample A treated with 0.66 wt % NH₄PF₆ in methanol    -   Sample D: Sample A treated with 0.76 wt % (NH₄)₃AlF₆ in water    -   Sample E: Sample A treated with 1 wt % H₃PO₄+0.66 wt % NH₄PF₆ in        methanol        -   Sample F: Sample A treated with 0.61 wt % NH₄BF₄ in methanol

The molarity of the fluorinated salt solutions was approximately2.5×10⁻³M in all cases.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention consists of certain novel features hereinafter fullydescribed, and illustrated in the accompanying drawings, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

FIG. 1 illustrates the powder X-ray diffraction patterns of:

-   -   a) an untreated 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂        electrode (Sample A);    -   b) a 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrode        treated with a 2.5×10⁻³M solution of NH₄PF₆ in methanol and        dried at 600° C. in air (Sample C);    -   c) a 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrode        treated with a 2.5×10⁻³M solution of (NH₄)₃AlF₆ in water and        dried at 600° C. in air (Sample D).

FIG. 2 illustrates the charge and discharge voltage profiles of lithiumhalf cells after the initial charge/discharge cycle, containingelectrode samples A to F between 3.0 and 4.6 V at 0.1 mA/cm² at roomtemperature,

FIG. 3( a-f) illustrates the charge and discharge voltage profiles ofthe 3^(rd) and 42^(nd) cycles of lithium half cells containing electrodesamples A to F between 3.0 and 4.6 V at 0.5 mA/cm² at room temperature.

FIG. 4 illustrates the capacity vs. cycle number of lithium half cellscontaining electrode samples A-F, between 3.0 and 4.6 V for the first 42cycles at room temperature.

FIG. 5 illustrates the capacity of lithium half cells containingelectrode samples A-E delivered between 4.6 and 3.0 V at current ratesbetween 0.16 and 8 mA at room temperature.

FIG. 6( a-e) illustrates the charge and discharge voltage profiles ofthe 3^(rd) and 102^(nd) cycles of lithium-ion (full) cells containingelectrode samples A, C, D, E and F between 3.0 and 4.6 V at 0.5 mA/cm²at room temperature.

FIG. 7 illustrates the capacity vs. cycle number of lithium half cellscontaining electrode samples A, C, D, E and F, between 3.0 and 4.6 V forthe first 100 cycles at room temperature.

FIG. 8 illustrates a schematic representation of an electrochemicalcell; and

FIG. 9 illustrates a schematic representation of a battery consisting ofa plurality of cells connected electrically in series and in parallel.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Metal oxide- or lithium-metal-oxide electrodes that provide a highelectrochemical potential, typically above 3 V, against lithium metal,such as oxides containing the first-row transition metal ions, V⁵⁺,Mn⁴⁺, Co⁴⁺ and Ni⁴⁺ ions tend to be strong oxidizing agents andtherefore can react with the non-aqueous electrolytes of lithium cells,particularly at the surfaces of electrode particles. For example, highlydelithiated layered Li_(1−x)MO₂ and spinel Li_(1−x)Mn_(2−y)M_(y)O₄electrodes can react spontaneously with the organic-based electrolytesolvents such as ethylene carbonate, diethyl carbonate or dimethylcarbonate; in extreme cases, the electrodes can release oxygen into thecell compartment that may cause possible thermal runaway, venting orexplosion, sometimes with fire. Even without the catastrophic failuredescribed above, the release of oxygen from the electrode lowers theaverage oxidation state of the electrochemically active transition metalions, particularly at the electrode surface, which can increase cellimpedance, and reduce the capacity and long term cycling stability ofthe cells. It is therefore important to find effective methods to reducethe high activity of charged metal-oxide- and lithium-metal-oxideelectrode surfaces without compromising the energy and power of thecells, while at the same time enhancing safety.

This invention relates, in general, to uncycled preconditionedmetal-oxide- or lithium-metal-oxide electrodes, including cathodesand/or anodes for non-aqueous lithium electrochemical cells andbatteries, the electrodes being preconditioned in an aqueous or,preferably, a non-aqueous solution containing stabilizing cations andanions, such as phosphorus, titanium, silicon, zirconium and aluminumcations and fluoride anions, that are chemically etched into the surfaceof the electrodes to form a protective layer in order to improve theelectrochemical properties of said cells and batteries and to methods ofmaking same. The invention relates, more specifically, to electrodesthat are preconditioned prior to cell assembly or in-situ in anelectrochemical cell to improve the capacity, cycling efficiency andcycling stability of lithium cells and batteries when charged to highpotentials. The invention relates, in particular, to metal oxide- andlithium-metal oxide electrode materials that in their unconditioned,charged state are strong oxidants.

In a first embodiment, the invention relates to preconditionedlithium-metal oxide electrodes selected from the family of layeredcompounds, LiMO₂, including lithium-rich materials, Li_(1+z)M_(1−z)O₂,that can be represented, alternatively, in two-component notation asxLi₂M′O₃.(1−x)LiMO₂ (0≦x<1) in which M′ is one or more metal ions withan average tetravalent oxidation state, selected preferably from Mn, Ti,and Zr, and in which M is one or more metal ions with an averagetrivalent oxidation state, and M is selected preferably from Mg, Al, Ti,V, Cr, Mn, Fe, Co, and Ni.

In a second embodiment, the invention relates to preconditionedlithium-metal oxide electrodes selected from the family of spinellithium-metal-oxides, LiM₂O₄, in which M is one or more metal cations,selected preferably from the subset of substituted spinellithium-manganese-oxides LiMn_(2−y)M_(y)O₄, in which M is one or moremetal ions selected preferably from Li, Mg, Al, Ti, V, Cr, Mn, Fe, Co,Ni, Cu and Zn, and two-component xLi₂M′O₃.(1−x)LiM₂O₄ (0≦x<1) compositeelectrodes in which M′ is one or more metal ions selected preferablyfrom Mn, Ti, and Zr. The relative amounts of M′ and M cations areselected such that there is charge balance in the electrode. ThexLi₂M′O₃.(1−x)LiMO₂ and xLi₂M′O₃.(1−x)LiM₂O₄ (0≦x<1) compositeelectrodes have complex and disordered structures, as described indetail by Thackeray et al. in J. Materials Chemistry, Volume 15, page2257, (2005) and references cited therein.

In a third embodiment, the invention relates to preconditionedmetal-oxide- or lithium-metal-oxide electrodes from the family ofV₂O₅-containing and MnO₂-containing compounds, such as V₂O₅ and MnO₂themselves, and lithium and silver derivatives thereof, such as LiV₃O₈(Li₂O.3V₂O₅), Ag₂V₄O₁₁ (Ag₂O.2V₂O₅), Li₂O.xMnO₂ and Ag₂O.xMnO₂ (x>0)compounds.

In a fourth embodiment, the invention relates to methods for fabricatingthe preconditioned metal-oxide- and lithium-metal-oxide electrodes bytreating the metal-oxide- and lithium-metal-oxide electrode particlesprior to cell fabrication and assembly with either an aqueous or anon-aqueous solution containing dissolved salts containing stabilizingcations and anions. In a preferred embodiment, the solutions are mildlyacidic, for example, with a pH between 4 and 7, preferably between 5 and7, and most preferably between 6 and 7. Because water reacts readilywith lithium at the negative electrode and can result in undesirableH⁺—Li⁺ ion-exchange reactions at lithium-metal-oxide electrodes, it ispreferable to precondition the electrodes in non-aqueous solutions, suchas alcohols, for example, methanol, ethanol and the like. Combinationsof aqueous and non-aqueous solvents for dissolving the salts can beused, for example, methanol and water. If aqueous solutions are used,then it stands to reason that the electrodes must be sufficiently heatedand dried to reduce the water content as much as possible withoutdamaging the electrochemical properties of the electrode. The inventionrelates more specifically to preconditioned metal-oxide- andlithium-metal-oxide electrode particles with surfaces etched bysolutions, preferably mildly acidic solutions with 4<pH<7, morepreferably 5<pH<7, and most preferably 6<pH<7, the solutions containingstabilizing ammonium, phosphorus, titanium, silicon, zirconium, aluminumand boron cations and fluoride anions, such as those found in NH₄PF₆,(NH₄)₂TiF₆, (NH₄)₂SiF₆, (NH₄)₂ZrF₆, (NH₄)₃AlF₆, NH₄BF₄ salts orderivatives thereof, to improve the capacity, cycling efficiency andcycling stability of lithium cells and batteries when charged to highpotentials. These preconditioning reactions can take place optionally inthe presence of lithium ions.

The following examples describe the principles of the invention andpossible methods for synthesizing the pre-reduced electrodes of thisinvention as contemplated by the inventors, but they are not to beconstrued as limiting examples.

EXAMPLES

Synthesis of 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ andPreconditioned 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.236)O₂Electrode Materials

Electrode materials with the formula0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ are prepared typicallyas follows. First, a Mn_(0.33)Ni_(0.33)Co_(0.33)(OH)_(x) precursor isprepared by coprecipitation of the required stiochiometric amounts ofmetal nitrates M(NO₃)₂.xH₂O (M=Mn, Ni, and Co). Li₂CO₃ is thenintimately mixed with the (Mn_(0.330)Ni_(0.335)Co_(0.335))(OH)_(x) (x˜2)precursor in a ratio ofLi₂CO₃:(Mn_(0.330)Ni_(0.335)Co_(0.335))(OH)_(x)=0.55:1 (orLi:(Mn+Ni+Co)=1.1:1). The powder mixture is calcined at 700° C. for 16hours in air and then at 950° C. for 12 hours in air to make0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂.

For the experiments of this invention, parent0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrode samples,referred to as Sample A, were preconditioned prior to cell assembly withvarious mild acids. For example, Sample A was treated with a 2.5×10⁻³MNH₄F solution in laboratory grade methanol containing trace amounts ofwater (typically up to 0.1%), the pH of which was approximately 6.5. Thesample was stirred in the solution at room temperature for 12 h and thendried (still under stirring) at ˜50° C., prior to heating at 600° C. inair for 6 hours (Sample B).

In a second example, Sample A was treated with a 2.5×10⁻³M NH₄PF₆solution in laboratory grade methanol containing trace amounts of water(typically up to 0.1%), the pH of which was approximately 6.5. Thesample was stirred in the solution at room temperature for 12 h and thendried (still under stirring) at ˜50° C., prior to heating at 600° C. inair for 6 hours (Sample C).

In a third example, Sample A was treated with a 2.5×10⁻³M (NH₄)₃AlF₆solution in water, the pH of which was approximately 6.5. The sample wasstirred in the solution at room temperature for 12 h and then dried(still under stirring) at ˜50° C., prior to heating at 600° C. in airfor 6 hours (Sample D).

In a fourth example, Sample A was treated with 1 wt % H₃PO₄ aqueoussolution together with a 2.5×10⁻³M NH₄PF₆ solution in laboratory grademethanol containing trace amounts of water (typically up to 0.1%), thepH of which was approximately 6.5. The sample was stirred in thesolution at room temperature for 12 h and then dried (still understirring) at ˜50° C., prior to heating at 600° C. in air for 6 hours(Sample E).

In a fifth example, Sample A was treated with a 2.5×10⁻³M NH₄BF₄solution in laboratory grade methanol containing trace amounts of water(typically up to 0.1%), the pH of which was approximately 6.5. Thesample was stirred in the solution at room temperature for 12 h and thendried (still under stirring) at ˜50° C., prior to heating at 600 ° C. inair for 6 hours (Sample F).

The X-ray diffraction patterns of Samples A, C and D are shown, by wayof example, in FIG. 1( a-c). There were no significant differences inthe X-ray patterns of Samples A, C and D, indicating that no significantchanges had occurred to the bulk structure of the individual compoundsduring the preconditioning reactions. The X-ray diffraction patterns ofSamples B, E and F were essentially identical to those of Samples A, Cand D.

Electrochemical Evaluation of0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ Electrodes andPreconditioned 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂Electrodes

Electrochemical evaluations of0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrodes andpreconditioned 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ positiveelectrodes were carried out as follows. The electrodes for the lithiumcells were fabricated from an intimate mixture of 84 wt % of0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrode powder (orpreconditioned 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂electrode powder), 8 wt % polyvinylidene difluoride (PVDF) polymerbinder (Kynar, Elf-Atochem), 4 wt % acetylene black (Cabot), and 4 wt %graphite (SFG-6, Timcal) slurried in 1-methyl-2-pyrrolidinone (NMP)(Aldrich, 99+%). An electrode laminate was cast from the slurry onto anAl current collector foil using a doctor-blade. The laminate wassubsequently dried, first at 75° C. for 10 h, and thereafter undervacuum at 70° C. for 12 h. The electrolyte was 1 M LiPF₆ in ethylenecarbonate (EC):ethylmethyl carbonate (EMC) (3:7 mixture). The electrodeswere evaluated at room temperature in lithium half cells (coin-type,size CR2032, Hohsen) with a lithium foil counter electrode (FMCCorporation, Lithium Division) and a polypropylene separator (Celgard2400). They were also evaluated at room temperature in full,lithium-ion-type coin cells against a MCMB 1028 graphite electrode.Cells were assembled inside an argon-filled glovebox (<5 ppm, H₂O andO₂) and cycled on a Maccor Series 2000 tester under galvanostatic modeusing a constant current density initially of 0.1 mA/cm² for the firsttwo cycles and, thereafter, at a higher current rate of 0.5 mA/cm².Lithium half cells were cycled between 4.6 and 3.0 V, whereaslithium-ion full cells were cycled between 4.5 and 3.0 V.

The charge/discharge voltage profiles of lithium half cells after theinitial charge/discharge cycle containing an untreated0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrode (Sample A)and 0.1Li₂MnO₃.0.9LiCO_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrodes that hadbeen preconditioned with mildly acidic solutions containing variousstabilizing cations and stabilizing fluorine anions (Samples B-F) areshown in FIG. 2( a-f), respectively. The figure demonstratesunequivocally that the initial discharge capacities of thepreconditioned electrodes (Samples B to E) are superior to that of theparent, unconditioned electrode (Sample A).

The charge and discharge voltage profiles of the 3^(rd) and 42^(nd)cycles of lithium half cells containing electrode samples A to F between4.6 and 3.0 V at 0.5 mA/cm² at room temperature are shown in FIG. 3(a-f), respectively. It is clear from these data that the preconditionedelectrodes (Samples B to F) provide enhanced capacity compared to theparent, untreated electrode (Sample A).

The cycling stability of untreated electrode (Sample A) andpreconditioned electrodes (Samples B-F) in lithium half cells arecompared graphically in capacity vs. cycle number plots in FIG. 4. It isclearly evident from the data that the preconditioned electrodes providesignificantly superior capacity and cycling stability to the parent,untreated electrode. The data also show that slightly superior cyclingstability is achieved from samples C, D, E and F that had beenpreconditioned with solutions containing stabilizing P, Al, and Bcations as well as NH⁴ cations and stabilizing F anions, compared toSample B that had been preconditioned with NH₄F. In this respect, it isnoted that any basic ammonium- or residual nitrogen-containing specieswill likely remain on the surface of the electrodes and may serve tocounter acid attack from the electrolyte, rather than being etched intothe electrode surface as occurs with the P, Al and B cations thatstabilize the electrode surface structure.

The capacity delivered by Samples A-E as a function of current rate isshown in FIG. 5. These data also clearly demonstrate the superiorelectrochemical properties of the preconditioned electrodes (SamplesB-E) that are able to withstand higher current discharge rates than theparent, untreated electrode (Sample A).

The charge and discharge voltage profiles of the 3^(rd) and 102^(nd)cycles of lithium-ion (full) cells containing electrode samples A, C, D,E and F between 4.5 and 3.0 V at 0.5 mA/cm² at room temperature areshown in FIG. 6( a-e), respectively; corresponding capacity vs. cyclenumber plots for the full 102 cycles are shown in FIG. 7. Theydemonstrate that significantly improved capacity is obtained from cellscontaining the preconditioned electrodes (Samples C-F) compared to theparent, untreated electrode (Sample A); moreover, the lithium-ion cellscontaining the preconditioned electrodes of the invention cycle withexcellent capacity retention/stability.

Electrolyte Additives

In a further embodiment of the invention, it was discovered that insteadof chemically preconditioning the electrodes with acid prior to cellassembly, the electrodes can be chemically conditioned, in situ, in anelectrochemical lithium cell by salts containing one or more cations ofammonium, phosphorus, titanium, silicon, zirconium, aluminum and boroncations and stabilizing fluoride anions, for example, NH₄PF₆,(NH₄)₂TiF₆, (NH₄)₂SiF₆, (NH₄)₂ZrF₆, (NH₄)₃AlF₆ and NH₄BF₄. Twolithium-ion cells were assembled containing an MCMB 1028 graphite anode,a 0.1Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ cathode and anelectrolyte comprising 1.2 M LiPF₆ in ethylene carbonate (EC):ethylmethyl carbonate (EMC). One of the cells contained 2 wt % NH₄BF₄ asan additive to chemically precondition the cathode surface in situ inthe electrochemical cell. The two cells were subjected to 3 formationcycles during which the cells were charged and discharged between 4.1and 3.0 V at ˜0.2 mA (˜C/10 rate). The cells were subsequently cycledand aged at an accelerated rate between 3.9 and 3.6 V at 55° C. at 2 mA(˜C/1 rate) for 2 weeks. The impedance of each cell was measured beforeand after the aging process at 3.72 V at room temperature. It wasobserved that the impedance growth of the cathode in the cell containingthe NH₄BF₄ electrolyte additive was significantly suppressed during theaging process, thereby providing evidence that the cathode surface hadbeen passivated, confirming the beneficial effects of preconditioningthe electrodes of this invention with mild acid, as describedhereinbefore.

The examples and results described in this application clearlydemonstrate the principles and advantages of this invention. It has beenshown, in particular, that superior electrochemical properties, forexample, enhanced capacity and cycling stability, can be obtained from0.1 Li₂MnO₃.0.9LiCo_(0.372)Ni_(0.372)Mn_(0.256)O₂ electrodes that arepreconditioned in aqueous or non-aqueous solutions containing bothstabilizing cations and anions, such as phosphorus, aluminum and boroncations and fluoride anions as well as ammonium ions, particularly whencells are cycled between 4.6 and 3.0 V. The superior electrochemicalproperties are attributed particularly to etched electrode surfaces thatcontain both stabilizing cations and anions, the stabilized surfacelayer being robust to the diffusion of lithium ions from theelectrode/electrolyte interface into the bulk of the electrode structureand vice-versa To those skilled in the art, it is easy to recognize thatthe principles of this invention in forming protective surfaces can beextended to other high potential metal-oxide- and lithium-metal-oxideelectrodes, such as the family of lithium-manganese-oxide spinels andV₂O₅-based or MnO₂-based electrode materials, as described herein. Thisinvention therefore relates to preconditioned metal-oxide andlithium-metal-oxide electrodes for both primary and secondary(rechargeable) lithium cells, a typical cell being shown schematicallyin FIG. 8, represented by the numeral 10 having a negative electrode 12separated from a positive electrode 16 by an electrolyte 14, allcontained in an insulating housing 18 with suitable terminals (notshown) being provided in electronic contact with the negative electrode12 and the positive electrode 16. Binders and other materials normallyassociated with both the electrolyte and the negative and positiveelectrodes are well known in the art and are not fully described herein,but are included as is understood by those of ordinary skill in thisart. FIG. 9 shows a schematic illustration of one example of a batteryin which two strings of electrochemical lithium cells, described above,are arranged in parallel, each string comprising three cells arranged inseries. The invention also includes methods of making the preconditionedelectrodes, cells and batteries including the same.

While there has been disclosed what is considered to be the preferredembodiments of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention. It is alsounderstood that additional improvements in the capacity and stability ofthe electrodes can be expected to be made in the future by improving andoptimizing the processing techniques whereby metal-oxide- andlithium-metal-oxide electrode materials are chemically etched in anaqueous or a non-aqueous solution containing stabilizing cations andanions to form a protective layer prior to their incorporation aselectrodes in electrochemical lithium cells.

1. A method of stabilizing a metal oxide or lithium-metal-oxideelectrode comprising contacting a surface of the electrode, prior tocell assembly, with an aqueous or a non-aqueous acid solution having apH greater than 4 but less than 7 and containing a stabilizing salt, fora time and at a temperature sufficient to etch the surface of theelectrode and introduce stabilizing anions and cations from the saltinto said surface, while the structure of the bulk of the electroderemains unchanged; wherein the stabilizing salt comprises fluoride andat least one cationic material selected from the group consisting ofammonium, phosphorus, titanium, silicon, zirconium, aluminum, and boron.2. The method of claim 1, wherein said stabilizing salt comprises one ormore of phosphorus, aluminum, and boron.
 3. The method of claim 1,wherein said electrode has the general formula of xLi₂M′O₃.(1−x)LiMO₂ inwhich M′ is one or more metal ions with an average tetravalent oxidationstate and 0≦x<1, and in which M is one or more metal ions with anaverage trivalent oxidation state.
 4. The method of claim 3, wherein M′is selected from Mn, Ti, and Zr and M is selected from Mg, Al, Ti, V,Cr, Mn, Fe, Co, and Ni.
 5. The method of claim 4, wherein M′ is Mn, andM is selected from Mn, Co and Ni.
 6. The method of claim 1, wherein saidelectrode has the general formula of xLi₂M′O₃.(1−x)LiM₂O₄ (0≦x<1) and M′is one or more metal ions with an average tetravalent oxidation stateand M is one or more metal cations with an average oxidation state of3.5.
 7. The method of claim 1, wherein said electrode comprises amaterial selected from the group consisting of a V₂O₅-containingcompound and a MnO₂-containing compound.
 8. The method of claim 1,wherein the pH of the aqueous or non-aqueous acid solution is greaterthan 6 but less than
 7. 9. The method of claim 1, wherein thestabilizing salt is selected from one or more of NH₄PF₆, (NH₄)₂TiF₆,(NH₄)₂SiF₆, (NH₄)₂ZrF₆, (NH₄)₃AlF₆, and NH₄BF₄.
 10. The method of claim1, wherein the non-aqueous acid solution comprises an alcohol.
 11. Themethod of claim 10, wherein the non-aqueous acid solution comprisesmethanol.
 12. A stabilized electrode made according to the method ofclaim
 1. 13. A method of stabilizing a metal-oxide orlithium-metal-oxide electrode comprising preconditioning the electrodein situ within a non-aqueous lithium cell comprising an electrolyte, bydissolving one or more salts selected from NH₄PF₆, (NH₄)₂TiF₆,(NH₄)₂SiF₆, (NH₄)₂ZrF₆, (NH₄)₃AlF₆, and NH₄BF₄ in the electrolyte priorto cell assembly and thereafter cycling the electrode through acharge-discharge cycle.
 14. A stabilized electrode prepared according tothe method of claim
 13. 15. A non-aqueous lithium electrochemical cellcomprising an anode, a lithium-containing electrolyte in a non-aqueousliquid solvent, and a cathode, wherein one or both of the anode andcathode are a stabilized electrode of claim
 12. 16. A non-aqueouslithium battery comprising a plurality of electrically connectedelectrochemical cells of claim
 15. 17. A non-aqueous lithiumelectrochemical cell comprising an anode, a lithium-containingelectrolyte in a non-aqueous liquid solvent, and a cathode, wherein oneor both of the anode and cathode are a stabilized electrode of claim 14.18. A non-aqueous lithium battery comprising a plurality of electricallyconnected electrochemical cells of claim 17.